<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<title>Snow</title>
<style type="text/css">
/* comment out - could also work
* {margin: 0; padding: 0;}
*/

body {
    /*You can use any kind of background here.*/
    background: #6b92b9;
}
canvas {
    /* could also work */
    /*display: block;*/
}
</style>
</head>

<body>

<div style=" background:#6b92b9; width:100%; height:2000px;" ></div>
<canvas id="canvas" style="position:fixed; top:0px;left:0px;z-index:80;pointer-events:none;"></canvas>

<script>
window.onload = function(){
    //canvas init
    var canvas = document.getElementById("canvas");
    var ctx = canvas.getContext("2d");
    
    //canvas dimensions
    var W = window.innerWidth;
    var H = window.innerHeight;
    canvas.width = W;
    canvas.height = H;
    
    //snowflake particles number 
    var mp = 3000; //max particles
    var particles = [];
    for(var i = 0; i < mp; i++)
    {
        particles.push({
            x: Math.random()*W, //x-coordinate
            y: Math.random()*H, //y-coordinate
            r: Math.random()*3+1, //radius
            d: Math.random()*mp //density
        })
    }
    
    //Lets draw the flakes
    function draw()
    {
        ctx.clearRect(0, 0, W, H);
        
       ctx.fillStyle = "rgba(255, 255, 255, 0.8)";
       /* ctx.fillStyle = "#FF0000";*/
        ctx.beginPath();
        for(var i = 0; i < mp; i++)
        {
            var p = particles[i];
            ctx.moveTo(p.x, p.y);
            ctx.arc(p.x, p.y, p.r, 0, Math.PI*2, true);
        }
        ctx.fill();
        update();
    }
    
    //Function to move the snowflakes
    //angle will be an ongoing incremental flag. Sin and Cos functions will be applied to it to create vertical and horizontal movements of the flakes
    var angle = 0;
    function update()
    {
        angle += 0.01;
        for(var i = 0; i < mp; i++)
        {
            var p = particles[i];
            //Updating X and Y coordinates
            //We will add 1 to the cos function to prevent negative values which will lead flakes to move upwards
            //Every particle has its own density which can be used to make the downward movement different for each flake
            //Lets make it more random by adding in the radius
            p.y += Math.cos(angle+p.d) + 1 + p.r/2;
            p.x += Math.sin(angle) * 2;
            
            //Sending flakes back from the top when it exits
            //Lets make it a bit more organic and let flakes enter from the left and right also.
            if(p.x > W || p.x < 0 || p.y > H)
            {
                if(i%3 > 0) //66.67% of the flakes
                {
                    particles[i] = {x: Math.random()*W, y: -10, r: p.r, d: p.d};
                }
                else
                {
                    //If the flake is exitting from the right
                    if(Math.sin(angle) > 0)
                    {
                        //Enter fromth
                        particles[i] = {x: -5, y: Math.random()*H, r: p.r, d: p.d};
                    }
                    else
                    {
                        //Enter from the right
                        particles[i] = {x: W+5, y: Math.random()*H, r: p.r, d: p.d};
                    }
                }
            }
        }
    }
    
    //animation loop
    setInterval(draw, 150);
}
</script>
</body>
</html>